Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Imagewise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element.
3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
Thermographic materials of type 1 can be rendered photothermographic by incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
Thermal dye transfer printing is a recording method wherein a dye-donor element is used that is provided with a dye layer wherefrom dyed portions or incorporated dyes are transferred onto a contacting receiver element by the application of heat in a pattern normally controlled by electronic information signals.
A survey of "direct thermal" imaging methods is given e.g. in the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography". Thermography is concerned with materials which are substantially light-insensitive, but are sensitive to heat or thermographic.
Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
A wide variety of chemical systems has been suggested some examples of which have been given on page 138 of the above mentioned book of Kurt I. Jacobson et al., describing the production of a silver metal image by means of a thermally induced oxidation-reduction reaction of a silver soap with a reductor.
According to U.S. Pat. No. 3,080,254 a typical heat-sensitive copy paper includes in the heat-sensitive layer a thermoplastic binder, e.g ethyl cellulose, a water-insoluble silver salt, e.g. silver stearate and an appropriate organic reducing agent, of which 4-methoxy-1-hydroxy-dihydronaphthalene is a representative. Localized heating of the sheet in the thermographic reproduction process, or for test purposes by momentary contact with a metal test bar heated to a suitable conversion temperature in the range of about 90.degree.-150.degree. C., causes a visible change to occur in the heat-sensitive layer. The initially white or lightly coloured layer is darkened to a brownish appearance at the heated area. In order to obtain a more neutral colour tone a heterocyclic organic toning agent such as phthalazinone is added to the composition of the heat-sensitive layer. Thermo-sensitive copying paper is used in "front-printing" or "back-printing" using infra-red radiation absorbed and transformed into heat in contacting infra-red light absorbing image areas-of an original as illustrated in FIGS. 1 and 2 of U.S. Pat. No. 3,074,809.
Examples of photothermographic materials are the so called "Dry Silver" photographic materials of the 3M Company, which are reviewed by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991.
U.S. Pat. No. 3,152,904 discloses an image reproduction sheet which comprises a radiation-sensitive heavy metal salt which can be reduced to free metal by a radiation wave length between an X-ray wave length and a five microns wave length and being distributed substantially uniformly laterally over said sheet, and as the image forming component an oxidation-reduction reaction combination which is substantially latent under ambient conditions and which can be initiated into reaction by said free metal to produce a visible change in colour comprising an organic silver salt containing carbon atoms and different from said heavy metal salt as an oxidizing agent and in addition an organic reducing agent containing carbon atoms, said radiation-sensitive heavy metal salt being present in an amount between about 50 and about 1000 parts per million of said oxidation-reduction reduction reaction combination.
Various methods of preparing substantially light-insensitive organic silver salts for use in such thermographic and photothermographic materials, as described in, for example, U.S. Pat. No. 2,910,377, U.S. Pat. No. 3,031,329, U.S. Pat. No. 3,458,544, U.S. Pat. No. 3,700,458, U.S. Pat. No. 3,960,908, U.S. Pat. No. 3,960,980, U.S. Pat. No. 4,193,804, U.S. Pat. No. 4,476,220, U.S. Pat. No. 3,839,049 and in Research Disclosure 17029 published in June 1978 and in references therein. A typical preparation process for the silver salts of fatty acids is described by D. Kloosterboer in "Imaging Processes and Materials, Neblette's 8th edition", edited by J. Sturge, V. Walworth and A. Shepp, page 279, Van Nostrand (1989) in which: "fatty acids are dispersed in water concentrations of 2 to 3%; heated above the titre point of the fatty acid (to about 80.degree. C.). With high speed stirring, the amount of sodium hydroxide equivalent to the desired percentage of silver soap is added. Silver nitrate is then added (to the whole quantity of sodium soap) to convert the sodium soap to the silver soap. During the silver addition the material forms a micelle with the hydrocarbon tail extended into the centre of the micelle. After the addition of the silver nitrate the solution may be filtered either hot or after cooling. The filtered solid is then dried until no further weight loss at about 50.degree. C. During drying the micelles collapse to form thin micelles approximately 1 .mu.m long, 0.1 .mu.m wide and 0.015 .mu.m thick". Such organic silver salts can also be produced by adding a silver salt, such as ammoniacal silver nitrate, silver trifluoroacetate, silver tetrafluoroborate, or silver oxide to a solution or dispersion of an organic compound with at least one ionizable hydrogen atom.
U.S. Pat. No. 3,458,544 describes a process for preparing water insoluble silver salt which comprises admixing a water immiscible phase containing an organic carboxylic acid dissolved therein, the silver salt of which is water insoluble, and an aqueous phase containing an alkali soluble silver complex having a dissociation constant higher than the silver salt of said organic acid, the pH of said aqueous phase being at least about 7.5, and recovering precipitated water insoluble silver salt of said organic carboxylic acid. The water insoluble silver salt forms as a precipitate at the interface of the two immiscible phases and is recovered, usually by settling, filtration, washing with distilled water to remove undesired anions and drying to produce a fine, free flowing powder having a high surface area and a high degree of purity.
GB-P 1,378,734 describes a process for producing a silver salt of an organic carboxylic acid having a grain size of less than 1 .mu.m in diameter and a grain which is almost spherical, which comprises mixing: (a) an aqueous solution of a silver nitrate or a silver complex with (b) a solution of an organic carboxylic acid in a solvent in which the organic carboxylic acid is soluble, both said silver salt of an organic carboxylic acid and silver nitrate being almost insoluble and with which water is sparingly miscible, so as to react said carboxylic acid with silver ions, the reaction being conducted in the presence of a soluble mercury compound and/or a soluble lead compound. According to example 12 of this patent, photothermographic materials with a silver coating weight of 1.0 g/m.sup.2, utilizing 0.3 .mu.m spherical grains of silver behenate particles obtained in the presence of mercuric nitrate, exhibited a transmittance of 85% at a wavelength of 500 nm.
In the prior art, substantially light-insensitive organic silver salts are produced by the addition of a silver salt or complex to an organic compound with an ionizable hydrogen atom, or its salt, such that initially a large excess of said acidic organic compound or its salt is present. The precipitation of such highly insoluble organic silver salts under such conditions makes the control of particle size and the avoidance of occluded acidic organic compound, or its salt, very difficult. Furthermore, particle agglomeration takes place, whether or not the particles are separated off and dried. Many properties of thermographic materials using said salts, are directly or indirectly dependent upon their particle size, e.g. light-insensitivity, storage properties, resolution, transparency and quantity of organic silver salt per unit area necessary to obtain the required maximum image density. Particle size is therefore important both as regards the economics of the production process and as regards the imaging properties of the thermographic or photothermographic material obtained with said particles.
Another important element in determining the imaging properties of such thermographic and photothermographic materials is the shape and morphology of the substantially light-insensitive organic silver salt-containing particles. With the exception of the process described in GB-P 1,378,734, the prior art processes for producing such organic silver salt particles or their suspensions produce needle-shaped particles such as, for example, silver behenate particles with a length of 1 .mu.m, a width of 0.1 .mu.m and a thickness of 0.01 .mu.m, as described by D. A. Morgan in "Handbook of Imaging Science", edited by A. R. Diamond, page 43, published by Marcel Dekker in 1991. Coating of dispersions of such needle-shaped particles using conventional techniques will produce alignment of such needles parallel to the coating direction resulting in material anisotropy, which will have an adverse effect on the imaging properties of such thermographic and photothermographic materials, particularly in the case of high resolution applications.
The incorporation of such ecologically suspect ions as mercury and lead ions into the organic silver salt particles, poses ecological problems in the disposal of such materials and of waste arising during the production process; and the handling thereof.
A still further important element in determining the imaging properties of such thermographic and photothermographic materials is the composition of the substantially light-insensitive organic silver salt-containing particles. With the single jet processes described in the prior art, a precise control over the microstructure of said particles is not possible when more than one molecular species is present in said particle.
The production economics of prior art thermographic and photothermographic materials are disadvantaged by the prior art production process for substantially light-insensitive organic silver salt grains and mixtures of substantially organic silver salt and photosensitive agent having to be carried out in several steps which can not be executed in a single reactor.